Introduction - Physical Gels from Biological and Synthetic Polymers - page 13

composition) of molecular precursors before investigating macroscopic properties. Most
of the techniques we discuss here are macromolecular or supramolecular (nano- or micro-
scale) in nature. The techniques we concentrate upon include scattering, ultramicroscopy
(cryomicroscopy and atomic force microscopy), differential scanning calorimetry
(DSC), rheology and simulations. The order in which these are discussed follows, at
least roughly, the so-called distance scale approach (Clark and Ross-Murphy, 1987 ).
Because of the convolution of factors, we assert that no single technique should be used
in isolation. In other words, in physical gel systems several parallel investigative
approaches are needed. Numerical simulations should provide strong support for exper-
imentalists to understand the origin of the effects observed and should also help to predict
what may happen when experimental conditions are changed.

Chapter 3 The sol-gel transition

In Chapter 3 we concentrate on a particular aspect that still occupies many chemists and
physicists. This is the very special limit when the solution switches from a liquid state,
where polymers are free to diffuse, to a solid-like state, where the polymers entrap the
solvent. This transition may happen quite suddenly, and this is what is de

-
gel transition. Despite the large number of theoretical and experimental publications
dealing with the sol

ned as the sol

-

gel transition, there is still a debate on the best way to determine the

precise nition of the transition

from sol to gel? Can we have a single method for all systems or is everyone free to choose
the most convenient way to describe their own particular system? In this chapter we present
the various ways of de

' gel-point ' . In other words, can there really be a unique de

ning the gel point from both the theoretical and the experimental
viewpoints, and put particular stress on both the problems and potential resolutions.

Chapter 4 General properties of polymer networks

Chapter 4 recalls some general properties ascribed to polymer networks. In ' chemical

gels or covalently cross-linked networks, the junctions are formed by chemical bonds

and entanglements; they are simply localized topological constraints. In

'

'
or physical networks, the junctions have complex structures, stabilized by secondary
forces (such as hydrogen bonding, hydrophobic interactions or labile ionic complexes).
Because in both cases macromolecules represent the backbone or the connecting paths of
the networks, there are many common features between chemical and physical gels,
including their elastomeric behaviour, which is fundamentally different from the classical
elasticity of crystals. In chemical gels at least, elastic constraints follow from their three-
dimensional network structure, and in turn determine that a polymer network can swell,
to a limited extent, but not dissolve. Here it is necessary to highlight the differences
between chemical and physical networks, since the latter have (more or less) labile
junctions. Another interesting feature appears in the rheology of physical gels, and is
related to the dynamics of junction formation and dissociation.
The succeeding chapters analyse various classes of physical gels according to their
respective formation mechanisms.

' physical gels